PURPOSE: To differentiate benign ovarian tumors in pregnancy from malignant ones using sonographic findings. MATERIALS & METHODS: Thirty nine patients who proved surgically to have ovarian tumors were investigated retrospectively from January to December 1996. Sonographic findings were evaluated in terms of the mass size, wall thickness, internal echogenicity, presence of septation and papillary projection in the cyst. RESULTS: Based on the sonographic evaluation, thirty eight cases were found to be benign ovarian tumor and one cases to be borderline malignancy. To be specific, there were fifteen dermoid cysts, nine mucinous cystadenomas, seven endometriotic cysts, four serous cystadenomas, two corpus luteal cysts, one fibroma and one papillary serous cystadenoma borderline malignancy. All cases except one fibroma appeared to have thin walled cystic mass and septim, and showed no significant difference in tumor size and internal echo density between benign and malignant ovarian tumors. But among three cases where small nodulations of size 0.4-1.9cm were observed, two were found to be mucinous cystadenoma and the other to be papillary serous cystadenoma borderline malignancy. CONCLUSION: The ovarian tumors tend to be more frequently observed in pregnancy, though they are benign ovarian tumor in most cases. Despite the low incidence of ovarian malignancy, the sonographic finding of papillary projections in evarian tumor hepls us to differentiate between benign and malignant tumors.